1. Field of the Invention
This invention relates to a Direct Access Storage Device (DASD) architecture for Count-Key-Data (CKD) records and, more specifically, to a CKD DASD system that uses Fixed-Block Architecture (FBA) records on an array of DASDs employing banding and separate read and write heads.
2. Description of the Related Art
Fixed-Block Architecture (FBA) is a common configuration used to format disk files. In a FBA formatted disk file, each concentric disk track is divided into a number of equal-sized sectors and each sector contains fixed-length blocks each having servo information, identification information (ID), and data. It is possible to eliminate the ID fields from FBA formatted devices by using the NO-ID technique described by Hetzler and Best in the copending patent application, "Sector Architecture For Fixed Block Disk File", filed on Jul. 10, 1991 as Ser. No. 07/727,680 and continued in application Ser. No. 08/082,826 filed on Jun. 23, 1993, and incorporated herein by this reference.
The IBM Count-Key-Data (CKD) architecture is a variable-length record configuration that is well-known in the art. The high-end IBM systems use CKD format with the MVS operating system. Each CKD record includes a fixed-length count field, an optional key field and a variable-length data field. The count field defines the length of the data field and the optional key field serves to identify the record. Each of the fields are distributed along a Direct Access Storage Device (DASD) track and are separated by gaps of predetermined fixed size. This gap, as it rotates under a read or write head at a constant speed, defines a time interval during which the system prepares to handle the next field.
Practitioners in the data storage arts have proposed numerous techniques for increasing DASD storage density and access efficiency. As DASD storage capacities increase, reliability continues to be an important consideration in the data storage arts. Moreover, as track spacing is reduced, read and write head alignment tolerances must be tightened considerably.
In the newer high-capacity DASDs, track widths are narrow and magneto-resistive heads may be used. Because there may be relatively significant offset between magneto-resistive read and write head centerlines arising from misalignment and head skew, provision is made for "micro-jogging" the common head actuator when switching from a read operation to a write operation. This actuator micro-jogging permits head realignment to compensate for centerline offset when switching from read to write after locating a target sector.
The micro-jogging operation can be understood as a small fractional-track adjustment to the radial location of the actuator assembly that is made when switching from a "read" operation to a "write" operation or back again. When reading, the actuator assembly is aligned so that the read head is precisely aligned with the center of the data track recorded on the rotating disk surface. When writing, the actuator assembly is moved slightly in a direction that aligns the write head centerline with the track. If a single head is used, micro-jogging may be necessary because of variation in the "effective" head centerline between read and write operations. The precise amount of this "micro-jog" depends on the head characteristics, the servo track characteristics, and other physical factors. Micro-jogging is typically accomplished by commands from the DASD controller.
In a CKD system, the DASD must first locate a count field (read) and then update the following data field (write). In newer high-density devices having magneto-resistive heads, the head actuator must be micro-jogged after the count field is read and before the data field is written. In a typical DASD, there is a gap of about 70 microseconds between the end of the count field and the beginning of a subsequent data field. Perhaps 300 to 500 microseconds are needed to perform such a microjog, which is many times longer than the inter-field gap delay. Thus, with such devices, either the inter-field gap delay must be increased by several-fold or a disk revolution is lost to provide for the necessary micro-jog time delay. The former option is very wasteful of disk capacity and the latter is very wasteful of disk access time and data transfer rate.
Newer DASDs also may be formatted in "bands" to exploit the increased linear capacity of tracks located at the larger radii. That is, the disk surface may be divided into a number of concentric "bands", each having a number of tracks formatted with a single physical track capacity. The track capacity in an interior band is less than the track capacity in an outer band for a given linear track data density. Thus, track capacity in a single disk could vary by several hundred percent without changing linear track density. Banded devices cannot be used with data formats that require a fixed track capacity unless provided with special operating system modifications.
R. A. Aziz ("Data Storage", IBM Technical Disclosure Bulletin, Vol. 20, No. 7, December 1977, pp. 2581-2583) proposes a banded spiral tracking format for increasing the storage disk recording capacity in a DASD. Aziz teaches the use of a single spiral segment within each concentric band that is entirely rewritten when updated but which may be repeatedly read in whole or part. Aziz asserts that his spiral technique doubles the data storage capacity of each disk compared to the nonbanded concentric tracking technique known in the art. However, his spiral track is impractical to edit and must be entirely overwritten when updated.
Practitioners have proposed a number of useful solutions to the continuing reliability problem in the data storage arts. As DASD storage capacity increases, so do the system-wide effects of single DASD failures. Generally, a DASD failure will shut down the entire system unless online backup data storage is provided. One elegant solution for this problem is known in the art as the Redundant Array of Inexpensive Disks (RAID) architecture described by David A. Patterson et al ("A Case for Redundant Arrays of Inexpensive Disks (RAID)"), ACM SIGMOD Conference, June 1988, pp. 109-116.
The data storage art is replete with techniques for emulating variable-length (e.g., CKD format) storage, while using FBA formatted DASDs. FBA DASDs are made more efficient the "NO-ID" technique disclosed in the above-cited Hetzler and Best patent application. CKD emulation is typically executed in a DASD control unit to make a FBA disk appear as a CKD disk to the host operating system. The introduction of RAID storage systems with parity has given rise to a clearly-felt need for CKD emulation methods for use in FBA formatted DASD arrays.
In U.S. Pat. No. 5,072,378, Paul S. Manka discloses a DASD RAID system with independently stored parity that is suitable for fault-tolerant storage of variable-length CKD records. Manka teaches the use of a separate DASD for storing parity blocks so that if one DASD fails or otherwise becomes unavailable, the affected record segments can be reconstructed by combining the remaining record segments with the parity blocks. Manka's system increases storage efficiency and accommodates variable-length data records without operating system modification by employing a virtual track imaging technique that interleaves logical sectors in physical track locations.
Menon discloses a system for accessing variable-length records stored on a synchronous array of FBA formatted DASDs in copending patent application "Method And Means For Execution of Commands Accessing Variable Length Records Stored On Fixed Block Formatted DASDs of an N+2 DASD Synchronous Array", filed on Apr. 2, 1991 as patent application Ser. No. 07/679,455 and appealed to the USPTO Board of Patent Interferences and Appeals on May 31, 1994, and entirely incorporated herein by this reference. Menon teaches a system for partitioning each variable-length CKD record into a variable number of equal fixed-length FBA blocks and synchronously writing the blocks in column-major order onto a DASD array. His column-major order is constrained so that the first block of each CKD record (the count field) is written at a one-column offset on the (N+1).sup.st DASD of the (N+2) DASD array. The last (N+2).sup.nd DASD is reserved for recording parity blocks to give fault-tolerance in a manner similar to that known in the art. However, Menon's system loses disk revolutions in situations where the head actuator must be micro-jogged when switching from read to write operations or back again. That is, the delay required to switch from read alignment to write alignment causes his system to miss the target field during the first disk revolution, requiring the system to wait until the target block again passes under the write head during a subsequent revolution. Also, Menon neither teaches nor suggests means for accommodating the variable track capacity introduced by the banded disk formats known in the art.
The above-cited Hetzler and Best patent application teaches a partial solution to the micro-jogging problem. Because no separate ID region is incorporated in the block header, the read-write recovery gap is eliminated along with the intra-block read-to-write transition for a FBA formatted disk. With earlier fixed block architectures, writing a data field requires prior reading of the ID field. Because it is not possible to complete a micro-jog between the ID and data fields, a disk revolution must be lost during the intra-block micro-jog. The NO-ID format disclosed by Hetzler and Best in the above-cited application eliminates this intrablock read-write transition. However, as noted above, Hetzler and Best discuss only FBA formats and neither consider nor suggest a method for handling variable length records such as the CKD format known in the art.
L. Levy ("Skewed Format Method For Personal Computer Hard Disks To Reduce Rotational Latency Delays", IBM Technical Disclosure Bulletin, Vol. 29, No. 4, September 1986, pp. 1867-8) suggests a skewed sector format to reduce rotational latency delays (lost revolutions) such as those introduced by inter-track seek operations or micro-jogging operations. Levy suggests offsetting sectors on adjacent concentric tracks by a predetermined angle . Levy's intent is to allow for cylinder-to-cylinder stepping time by introducing a passive rotational latency between tracks. However, Levy neither discloses nor suggests any useful technique for introducing rotational latency in RAID systems involving synchronous disk arrays where each logical record is spread over many DASDs's.
No efficient CKD-emulation technique is known in the art for arrays of FBA-formatted banded DASDs having different read and write head alignments. The related unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.